Whenever a new 5x5 is released, it is often only a matter of time before a new 4x4 is released too. Although that did not happen for the DaYan NeZha which was rather strange.
modern 4x4 mechanisms essentially all function like 5x5s with invisible edge pieces, this design works far better than the original design seen on the Rubik's Revenge 4x4 which featured some sort of ball core mechanism.
Gan have recently released a 5x5 and I expect to receive stock of it very soon, as of writing this blog post, it is currently up for pre-order but it seems that the 4x4 is already announced and likley to be available to the public in a few months time. We can assume they will have to release this over the summer because Gan always release their new flagship 3x3 in around september/october time each year.
In 2018, Gan released their first 4x4 and unfortunately it was not very good, it was slow and uneven on the different layers but it looks like this time Gan have learnt their lesson and are now able to design big cubes that perform well, starting with their 5x5 first. I refused to ever stock the original Gan 4x4 but am now excited to be able to say there is a decent Gan 4x4 available whenever anyone asks.
We have seen the initial testing videos from Leo Borromeo and Matty Hiroto Inaba and both cubers seem to be getting very good solve times.